1. Field of the Invention
The present invention relates to a method for transmitting data in a communication system, and more specifically, a process and circuits for transmitting information using multiple antennas transmission diversity scheme.
2. Description of the Related Art
A typical cellular radio system includes a number of fixed base stations and a number of mobile stations. Each base station covers a geographical area, which is defined as a cell.
Typically, a non-line-of-sight (NLOS) radio propagation path exists between a base station and a mobile station due to natural and man-made objects disposed between the base station and the mobile station. As a consequence, radio waves propagate while experiencing reflections, diffractions and scattering. The radio wave which arrives at the antenna of the mobile station in a downlink direction, or at the antenna of the base station in an uplink direction, experiences constructive and destructive additions because of different phases of individual waves generated due to the reflections, diffractions, scattering and out-of-phase reccombination. This is due to the fact that, at high carrier frequencies typically used in a contemporary cellular wireless communication, small changes in differential propagation delays introduces large changes in the phases of the individual waves. If the mobile station is moving or there are changes in the scattering environment, then the spatial variations in the amplitude and phase of the composite received signal will manifest themselves as the time variations known as Rayleigh fading or fast fading attributable to multipath reception. The time-varying nature of the wireless channel require very high signal-to-noise ratio (SNR) in order to provide desired bit error or packet error reliability.
The scheme of diversity is widely used to combat the effect of fast fading by providing a receiver with multiple faded replicas of the same information-bearing signal.
The schemes of diversity in general fall into the following categories: space, angle, polarization, field, frequency, time and multipath diversity. Space diversity can be achieved by using multiple transmit or receive antennas. The spatial separation between the multiple antennas is chosen so that the diversity branches, i.e., the signals transmitted from the multiple antennas, experience fading with little or no correlation. Transmit diversity, which is one type of space diversity, uses multiple transmission antennas to provide the receiver with multiple uncorrelated replicas of the same signal. Transmission diversity schemes can further be divided into open loop transmit diversity and closed-loop transmission diversity schemes. In the open loop transmit diversity approach no feedback is required from the receiver. In one type of closed loop transmit diversity, a receiver knows an arrangement of transmission antennas, computes a phase and amplitude adjustment that should be applied at the transmitter antennas in order to maximize a power of the signal received at the receiver. In another arrangement of closed loop transmit diversity referred to as selection transmit diversity (STD), the receiver provides feedback information to the transmitter regarding which antenna(s) to be used for transmission.
An example of open-loop transmission diversity scheme is the Alamouti 2×1 space-time diversity scheme. The Alamouti 2×1 space-time diversity scheme contemplates transmitting a Alamouti 2×2 block code using two transmission antennas using either two time slots (i.e., Space Time Block Code (STBC) transmit diversity) or two frequency subcarriers (i.e., Space Frequency Block Code (SFBC) transmit diversity).
One limitation of Alamouti 2×1 space-time diversity scheme is that this scheme can only be applied to two transmission antennas. In order to transmit data using four transmission antennas, a Frequency Switched Transmit Diversity (FSTD) or a Time Switched Transmit Diversity (TSTD) is combined with block codes. In case of combined SFBC+FSTD scheme or STBC+TSTD scheme, the matrix of the transmitted symbols from the four transmission antennas may be given as:
                              [                                                                      T                  11                                                                              T                  12                                                                              T                  13                                                                              T                  14                                                                                                      T                  21                                                                              T                  22                                                                              T                  23                                                                              T                  24                                                                                                      T                  31                                                                              T                  32                                                                              T                  33                                                                              T                  34                                                                                                      T                  41                                                                              T                  42                                                                              T                  43                                                                              T                  44                                                              ]                =                  [                                                                      S                  1                                                                              S                  2                                                            0                                            0                                                                                      -                                      S                    2                    *                                                                                                S                  1                  *                                                            0                                            0                                                                    0                                            0                                                              S                  3                                                                              S                  4                                                                                    0                                            0                                                              -                                      S                    4                    *                                                                                                S                  3                  *                                                              ]                                    (        1        )            where Tif represents symbol transmitted on the ith antenna and the jth subcarrier or jth time slot, and i=1,2,3,4, j=1,2,3,4 for the case of four transmission antennas, where S1, S2, S3 and S4 are symbols to be transmitted.
The problem with combined SFBC+FSTD scheme and STBC+TSTD schemes is that only a fraction of the total transmission antennas and hence power amplifier capability is used for transmission in a given frequency or time resource. This is indicated by ‘0’ elements in the SFBC+FSTD and STBC+TSTD matrix given above. When the transmit power on the non-zero elements in the matrix is increased, bursty interference is generated to the neighboring cells degrading system performance. Generally, bursty interference manifests itself when certain phases of a frequency hopping pattern incur more interference than other phases.